E-commerce platforms operate at the intersection of trust, scale, and speed. While digital marketplaces enable unprecedented participation, they also create persistent exposure to fraud, abuse, and manipulation. This chapter frames risk management as foundational infrastructure rather than a peripheral compliance function. It surveys major categories of platform risk-identity abuse, account takeover, transaction fraud, incentive exploitation, seller misconduct, payment loss, and coordinated manipulation-and explains why these threats arise as rational responses to platform incentives. Because adversaries adapt rapidly and scale magnifies impact, static rule systems alone are insufficient. Machine learning becomes essential for interpreting high-dimensional behavioral signals under real-time constraints and shifting attack patterns. The chapter’s organizing thesis is that risk must be managed as an integrated system spanning data, modeling, decisioning, monitoring, and governance. It also clarifies the roles of rules, models, and human operations and motivates why risk must be designed for adversarial pressure, not average users. The chapter sets the language used throughout the book to connect technical choices to business and customer outcomes.